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Oct. 18, 1949.
Filed May 29, 1947
B401 12 652552
Patented Oct. 18, 1949
» 2,485,045
Paul D. Gerber, Woodlynne, N. J., assignor to
Radio Corporation of America, a corporation
of Delaware
Application May 29, 1947, Serial No. ‘751,383
6 Claims.
My present invention relates to piezoelectric
oscillators and particularly to improvements in
oscillating circuits and systems wherein the crys
tal is mounted, without any clamping force, be
(01. 250-36)
diagram showing a crystal controlled oscillator
incorporating my invention.
'As previously indicated, the quartz or other
piezoelectric crystal X employed for controlling
tween two or more spaced apart electrode plates. 5 the frequency of a radio broadcast or other oscil-‘
Piezo-electric crystals that have been cut to
lating circuit operating at a frequency of less
oscillate at a frequency lower than 2000 kilocycles
than 2000 kc. is almost invariably placed without
per second are extremely sensitive to any force
clamping pressure upon the bottom electrode B
which tends to limit their freedom of movement.
of a simple air-gap holder H having retaining
Accordingly, the prior art dictates the use of so 10 pins l, 2, 3 and 4 positioned, say, adjacent two
called “simple air-gap” holders for the mounting
diagonally opposite corners of the crystal, for
of such crystals, and many ingenious ways have
limiting the degree of lateral movement of the
been devised for minimizing even the small damp
crystal. It can be demonstrated that when such
ing effect necessarily present in such holders due
a crystal unit is connected to a direct-current
(a) to friction between the crystal and the elec
potential (applied through the leads a and D)
trode surface uponwhich the crystal rests (see
such that the upper electrode A is positive and
Smalts 2,231,483 and Gerber 2,284,088) and (1))
the lower electrode B is ‘negative in potential,
due to the presence of the “pins” which limit
then, the top electrode A will immediately acquire
the extent of the lateral movement of the crystal
numerous positive (+) charges and the lower
upon the said electrode surface (as to this see, 20 electrode B negative (—) charges, with a result
Clark 2,139,998 and Bokovoy 2,285,143).
ing force of attraction between them. Given
In spite of the foregoing mechanical precau
time, some negative charges will appear on the
tions, and in spite of certain improved electrical
top surface of the crystal as if in an eiTort to
features of construction (e. g. Smalts 2,139,918),
effect a conducting circuit external to the source
incidences have arisen wherein air-gap mounted 25 of potential difference. Because of the dielectric
crystals failed to exhibit the stability of perform
ance required in the operation of oscillators de
signed for use in radio broadcast and other sys
tems for the communication of intelligence. In
studying such unsatisfactory crystal units I have '
discovered that their breakdown or faulty per
formance resulted from, or was accompanied by,
and insulating properties of'the crystal, and the
relatively small air-gap thereabove, it is difilcult
exactly to visualize the distribution of the said
electrical charges.
However, eventually these
charges appear on the top surface of the crystal
in a quantity su?icient to yield a force of attrac
tion great enough to overcome the gravitational
vertical displacement of the crystal in its air-gap.
pull on the crystal. Asa result, the crystal will
In no case could this vertical movement be attrib
be lifted from the bottom electrode and produce
uted to shocks or tremors of external origin and
a jump or shift in operating frequency. If by
I have traced the fundamental cause of the said
contact with the top electrode, or by interruption
undesired movement of the crystal to the accu
of operation, the charges are neutralized or
mulation of static charges of opposite polarity
otherwise removed from the top surface of the
upon the upper electrode of the holder and upon
crystal so that it drops back into normal oper
that face of the crystal which is presented (across 40 ating position on the lower electrode, the above
the air-gap) to the said electrode.
described operating cycle will be repeated. It is
Accordingly, the principal object of my inven
of course apparent that the same phenomenon
tion is to provide a method of, and means for,
can also take place with the D.-C. potential re
preventing or reducing to a negligible amount,
45 versed in polarity and, it is logical to assume,
the accumulation of static charges in crystal _ with crystals other than quartz.
oscillator installations of the type wherein the
In many present day crystal controlled broad
crystal is contained in a so called “air-gap" type
of holder.
cast transmitters with which I am familiar the
crystal unit is subjected to a direct-current po
My invention will be described in connection
with the accompanying drawing wherein Fig. 1
tential capable of producing the above described
unstable operating conditions. At ?rst glance it
is a side-elevational view of a conventional air
might appear that the undesirable behavior of
the crystal might be cured by reducing the D.-C.
certain parts being marked with electrical sym
potential applied to the vacuum tube but this is
bols which will be referred to in describing the 55 precluded, at least in the broadcast ?eld, by the
problem here involved, and Fig. 2 is an, electrical
fact that the power output of some if not all of
gap holder containing a piezoelectric crystal;
the many different types of oscillators would be
impaired by such a direct procedure. It is thus
apparent that the desired result must be achieved
without adversely affecting either the vibratile
when the leakage resistance of the capacitor C
was of the order of 1000 megohms the D.-C.vo1t
age appearing across the crystal was substan
tially 1 percent of 111 volts or 1+volts. This
relatively minute D.-C. voltage across the spaced
apart electrodes A and B of the air-gap holder H
did not permit the accumulation of static charges
a and b.
of su?ioient magnitude to lift the crystal off its
The foregoing and other requirements are met
bottom electrode and hence stabilize the vibratile
in accordance with my intention by introduc
ing a resistance-capacitive (R.-C.) network in 10 performance of the crystal and the output of the
performance of the crystal or of the oscillating
circuit which is connected thereto at the points
the circuit of the oscillator on the side of the
terminals a and b which lies adjacent (elec
is shown as comprising a blocking condenser C
in series with the crystal and a leakage resistor
tentials, of a capacitor having one terminal con
nected to one side of said piezoelectric element
and the other terminal connected to a point in
the oscillatory circuit common to said D.-C. po
I claim as my invention:
.1. The combination with a piezoelectric oscil
trically) to the crystal unit, as shown in Fig;
the type wherein the piezoelectric ele
2. This network, which may be incorporated
in any conventional crystal controlled oscillator, 15 ment is normally subjected to direct current po
R. in shunt with the crystal. With the capacitor
C and resistor R, thus arranged the D.-C. poten
tial present across the terminals a and b will be 20 tentials thereby effectively isolating all said D.-C.
potentials from said piezoelectric element except
divided between R and C as determined by the
for potentials due to leakage through said capac
ratio of the leakage resistance of the condenser
itor, and a resistor connected in shunt with only
C and the ohmic resistance of the resistor B.
said piezoelectric element to provide a path for
(This assumes that the D.-C. resistance of the
said leakage thereby minimizing the accumula~
air-gap is substantially in?nite.) This ratio:
tion of charge on said element.
_C_ (leakage resistance)
R (rated ohmic resistance)
for a speci?c magnitude of D.-C. potential be
tween a and b must be suf?ciently high to prevent
an accumulation of charges on the upper elec
trode (and on the adjacent face of the crystal)
su?icient to lift the crystal off the bottom elec
trode. The leakage resistance of the well de
signed condensers now commercially available is
2. The invention as set forth in claim 1 and
wherein said oscillator comprises a vacuum tube
having a grid resistor in circuit therewith and
wherein said direct current potential is derived
at least in part from said grid resistor.
3. The invention as set forth in. claim 1 and
wherein said oscillator comprises a vacuum tube
having an anode and a source of energizing cur
10 megohm resistor the resulting ratio of 100 to 1,
or higher, has proved entirely satisfactory for
rent connected to said anode, and wherein said
direct current potential is derived at least in part
from said anode supply.
4. The invention as set forth in claim 1 and
present day broadcast oscillator circuits.
wherein the leakage resistance of said capacitor
at least 1000 megohms, and when R comprises a
is substantially no less than 1000 megohms and
the resistance of said resistor is of the order of
10 megohms.
whether the D.-C. potential to which the crystal
5. The invention as set forth in claim 1 and
is subjected is derived from a grid resistor, di
wherein said piezoelectric element comprises a
rectly from the plate current source, or from both.
In the particular embodiment of my invention 45 quartz crystal cut to respond to a frequency
substantially no higher than 2000 kilocycles per
which has been selected for purposes of illustra
second and wherein said quartz crystal is mounted
tion the electrodes A and B for the crystal X are
in an air-gap type of holder.
connected respectively to the anode 5 through
6. The invention as set forth in claim 1 and
the capacitor C, and to the grid 6 of a three ele
ment vacuum tube 1. The anode 5 is connected 50 wherein the ratio of the leakage resistance of
said condenser to that of the ohmic resistance
to the positive terminal (-1-) of a source of direct
of said resistor is suf?ciently high to prevent the
current through a tank circuit 8, and there is a
accumulation of static charges in said holder.
grid resistor 9 connected between cathode l0 and
grid 6. In this case the direct current potential
to which the crystal is subjected comprises a
combination of the D.-C. potential from the
anode supply, less the D.-C. potential developed
The following references are of record in the
My invention may be applied to crystal con
trolled oscillators of various types, irrespective of
across the grid resistor 9.
In one typical case
file of this patent:
wherein the D.-C. potential applied to the anode
was 165 volts and the potential across the grid
resistor 9 was 54 volts the net potential between 60 Number
the anode and grid was 111 volts.
An appro
priate value for the blocking condenser C in this
case was 6500 mmfd. and the value of the leak
age resistor R was of the order of 10 megohms.
Thus, in agreement with the formula
Kinn ____________ __ Dec. 15, 1936
Hight ____________ __ Apr. 5, 1938
Hanseli __________ __ Nov. 22, 1938
Parker ___________ __ Oct. 13, 1942
Usselman et al ____ __ Nov. 26, 1946
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